What shapes the oral jaws? Accommodation of complex dentition correlates with premaxillary but not mandibular shape.

BACKGROUND: Teeth are integrated into the vertebrate oral jaws to provide a functional unit for feeding, however little is known about how this integration occurs during growth and development. The purpose of this study is to identify the ontogenetic changes in oral jaw shape that are associated with the transition of the oral dentition from unicuspid teeth to multicuspid teeth. Here, we compare the shape of the occluding upper (premaxilla) and lower (mandible) jaws of the toothed Mexican tetra (Astyanax mexicanus) and the toothless (oral teeth present, pharyngeal teeth absent) zebrafish (Danio rerio) over development. Gross morphology combined with morphometric analyses were used to analyse shape changes of the occluding oral jaws in each species. Histological analyses were also used to examine the development of the mandibular symphysis. RESULTS: The occluding edge of the premaxilla is the first region to ossify in the Mexican tetra, but the last to ossify in zebrafish. Morphometric analyses revealed that the early shape of the premaxillae (in fish younger than 8mm SL) is the same in each species but that the premaxilla shape changes significantly at larger sizes. These changes are apparent in the tooth bearing region of the Mexican tetra. The rostral region of the mandible also houses teeth, however ossification and shape in this region were surprisingly similar between species despite differences in the presence of oral dentition. Furthermore, we found that the mandibular symphysis of the Mexican tetra is composed of interdigitating bone, while the symphyseal region of the zebrafish is composed of fibrous connective tissue. CONCLUSIONS: These differences in the jaw skeleton have likely evolved due to different feeding strategies utilised by each species. Our results show that premaxillae shape correlates strongly with the development of complex dentitions unlike in the mandible. This study provides important insights into the relationship between jaw and tooth development in bony fishes and suggests that these mechanisms may be similar amongst vertebrates.

Scale and tooth phenotypes in medaka with a mutated ectodysplasin-A receptor: implications for the evolutionary origin of oral and pharyngeal teeth.

Ectodermal contribution to the induction of pharyngeal teeth that form in the endodermal territory of the oropharyngeal cavity in some teleost fishes has been a matter of considerable debate. To determine the role of ectodermal cell signaling in scale and tooth formation and thereby to gain insights in evolutionary origin of teeth, we analyzed scales and teeth in rs-3 medaka mutants characterized by reduced scale numbers due to aberrant splicing of the ectodysplasin-A receptor (edar). Current data show that, in addition to a loss of scales (83% reduction), a drastic loss of teeth occurred in both oral (43.5% reduction) and pharyngeal (73.5% reduction) dentitions in rs-3. The remaining scales of rs-3 were irregular in shape and nearly 3 times larger in size relative to those of the wild-type. In contrast, there was no abnormality in size and shape in the remaining teeth of rs-3. In wild-type medaka embryos, there was a direct contact between the surface ectoderm and rostral endoderm in pharyngeal regions before the onset of pharyngeal tooth formation. However, there was no sign of ectodermal cell migration in the pharyngeal endoderm and hence no direct evidence of any ectodermal contribution to pharyngeal odontogenesis. These data suggest differential roles for Eda-Edar signaling in the induction and growth of scales and teeth and support the intrinsic odontogenic competence of the rostral endoderm in medaka.